In general, part-time four-wheel drive transfer cases used on vehicles equipped deliver driving power normally only to front or rear axle assembly nd, if desired, to both the front and rear axle assemblies. When the vehicle with its part-time four-wheel transfer case shifted into a two-wheel drive mode is in motion, gears of an axle differential case in association with non-driven wheels and a transfer case connected to the axle differential are forced to turn by the non-driven wheels, causing a torque loss.
In an attempt at avoiding such a torque loss caused in the part-time four-wheel transfer case, a free-wheeling device, with its associated clutch means, is incorporated in an axle shaft between one of the non-driven wheels and an axle differential associated with the non-driven wheels to split the axle shaft when the transfer case is in the two-wheel drive mode so as to disconnect the transmission of motion from the non-driven wheels to the axle differential. Owing to the provision of such a free-wheeling device, the transfer case is independent of motion of the non-driven wheels. One such a free-wheeling device is disclosed in, for example, U.S. Pat. No. 4,381,828 entitled "Vehicle Drive System Having First and Second Operating Modes And Method of Operating Same", issued May 3, 1983.
Meanwhile, in vehicles equipped with four-wheel drive having a center-differential which is shiftable into a differential-locked drive mode in which no differential motion is caused between front and rear wheels, the differential-locked drive mode is less frequently used than other drive modes such as a differential-free drive mode and a two-wheel drive mode. Specifically, the differential-locked drive mode is generally used to certainly transmit driving power from the engine to wheels in such a case that, for example, one of traction wheels sticks while the vehicle is running in the differential-free four-wheel drive mode. In consideration of this, it is desirable to adapt drive mode selecting means such as a manual shift lever operated by a driver to have an operated position selecting a two-wheel drive mode which is most frequently used adjacent to an operated position selecting a differential-free drive mode which has a low frequency in use.
In a case of a differential lockable transfer case equipped with a free-wheeling device, when shifting the transfer case into the four-wheel differential-free drive mode from the two-wheel drive mode, the center-differential is possibly shifted into the four-wheel differential-free mode while the free-wheeling device is not yet clutched. If the free-wheeling device is unclutched when vehicle is traveling in the four-wheel drive mode, because the free-wheeling device disconnects the transmission of driving power, the vehicle is possibly led into an unstable control of operation.